With the development of information communication technologies, a variety of wireless communication technologies have been developed. Among these technologies, wireless local area network (WLAN) is a technology that Internet access is possible in a wireless way in homes, business or specific service providing areas, using portable terminal such as personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), or the like, based on wireless frequency technologies.
WLAN technologies is created and standardized by the IEEE 802.11 Working Group under IEEE 802 Standard Committee. IEEE 802.11a provides a maximum PHY data rate of 54 Mbps using an 5 GHz unlicensed band. IEEE 802.11b provides a maximum PHY data rate of 11 Mbps by applying a direct sequence spread spectrum (DSSS) modulation at 2.4 GHz. IEEE 802.11g provides a maximum PHY data rate of 54 Mbps by applying orthogonal frequency division multiplexing (OFDM) at 2.4 GHz. IEEE 802.11n provides a PHY data rate of 300 Mbps using two spatial streams and bandwidth of 40 MHz, and provides a PHY data rate of 600 Mbps using four spatial streams and bandwidth of 40 MHz.
As such WLAN technology becomes more prevalent and its applications become more diverse, there is increasing demand for new WLAN technology that can support a higher throughput than IEEE 802.11n. Very high throughput (VHT) WLAN technology, that is one of the IEEE 802.11 WLAN technologies, is proposed to support a data rate of 1 Gbps and higher. IEEE 802.11ac has been developed as a standard for providing VHT in the 5 GHz band, and IEEE 802.11ad has been developed as a standard for providing VHT in the 60 GHz band.
In a system based on such WLAN technology, an access point (AP) sets bits corresponding to an association ID (AID) of each terminal in a bitmap control and a partial virtual bitmap among traffic indication map (TIM) elements of a beacon when there is data to be transmitted to the terminal, whereby the terminal can determine whether there is data to be transmitted.
Since a length field of a TIM element format has 1 octet, the partial virtual bitmap may have up to a maximum of 251 octets, and therefore a total of 2007 terminals (8251-1) can be expressed.
Meanwhile, in order to express and manage at least 2007 terminals as a TIM bitmap, a method of encoding the TIM bitmap in units of blocks while having a hierarchical AID structure has been proposed.
Here, a page ID is displayed in an existing bitmap control, and encoded bitmap blocks are continuously displayed in units of blocks in the partial virtual bitmap.
In addition, a single encoded bitmap block includes a block control of 3 bits, a block offset of 5 bits, a fixed length part of 2 octets including a block bitmap of 1 octet, and a sub-block bitmap part having a variable length.
However, in order for a terminal in an N-th block (N is a natural number greater than 1) to determine whether a TIM bit of the terminal is set, a position of a block control of the N-th block has to be found, and an actual position of the bitmap has to be determined in accordance with a block encoding mode. In addition, since each block has a variable length, a position of an N-th block control cannot be found from a fixed position, and blocks from a first block to an (N−1)-th block have to be sequentially decoded.
That is, when there is actual data to be transmitted, bitmaps of all blocks should be all decoded, but in an environment in which a small amount of data is intermittently transmitted, such as a sensor network, there is a problem in that all blocks have to be decoded with respect to all delivery traffic indication map (DTIM) beacons in order to determine the presence or absence of data even though there is no data.